Low: libcrmcommon: NULL-check strdup() in pcmk__register_format()

Dereferencing null pointer found by cppcheck.

Return ENOMEM instead of asserting since this is a library function that
returns a standard Pacemaker return code. However, nothing checks it,
and it would have segfaulted in the past, so it might be better to
assert here.

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>